Merge pull request #4499 from LLK/multi-upload

src/components/action-menu/action-menu.jsx

src/components/sprite-selector/sprite-selector.jsx

src/components/stage-selector/stage-selector.jsx

src/containers/costume-tab.jsx

src/containers/sound-tab.jsx

src/lib/file-uploader.js

test/fixtures/movie.wav

test/fixtures/sneaker.wav

test/integration/backdrops.test.js

test/integration/costumes.test.js

test/integration/sounds.test.js

test/integration/sprites.test.js